This invention relates to a novel process for the purification of acetic acid which has been formed by the carbonylation of methanol or methyl acetate in the presence of a Group VIII metal carbonylation catalyst. More specifically, this invention relates to a novel process for removing alkane impurities from acetic acid formed by Group VIII metal catalyzed carbonylation processes.
Among currently-employed processes for synthesizing acetic acid one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
An improvement in the prior-art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in copending, commonly assigned application U.S. Pat. No. 5,001,259, issued Mar. 19, 1991 and European patent application 161,874; published Nov. 21, 1985. As disclosed therein acetic acid is produced from methanol in a reaction medium comprising methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically-effective concentration. The invention therein resides primarily in the discovery that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 10 wt. % or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt. % or 15 wt. % water) by maintaining in the reaction medium, along with a catalytically-effective amount of rhodium, at least a finite concentration of water, methyl acetate, and methyl iodide, a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The applications teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt. %, so low that it can broadly be defined simply as "a finite concentration" of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. resistance to catalyst precipitation, especially during the product-recovery steps of the process wherein distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Ser. No. 870,267 is herein incorporated by reference.
The acetic acid which is formed by the carbonylation of methanol is converted to a high purity product by conventional means such as by a series of distillations.
During the manufacture of acetic acid by the previously described methods one group of impurities found in the crude product is alkanes manufactured in the carbonylation process. This formation of alkanes was recognized by Price. His invention, described and claimed in U.S. Pat. No. 4,102,922, involved removing the alkanes by stripping the acidic products of the reaction away from the alkanes after removal of the catalyst. The reaction mixture is carried to a pressure let-down vessel denoted as a flasher where the products are vaporized and removed from a residue of catalyst. The catalyst is recycled to the reactor. The flashed product containing methyl iodide, water, acetic acid, and the alkanes is fed to a splitter and allowed to separate into at least two liquid phases, one phase containing acetic acid and water which is returned to the reactor and a second phase denoted herewithin as a heavy phase. To effect removal of the alkanes, a slipstream of the heavy phase from the splitter column is stripped using carbon monoxide as a stripping gas, removing the alkanes as the bottoms stream from the latter distillation.
We have discovered a method by which the alkanes can be removed according to the method of Price but with the unexpected benefit of realizing the recovery of acetic acid from the heavy phase that would normally be discarded by the practice of the prior art.
In a preferred embodiment of our invention we have found a method of effecting the alkanes removal at low-water conditions wherein the water balance in the reaction system is maintained.